1. Field of the Invention
The present invention relates to a delay detonator, and more especially to a detonator adapted to be used in millisecond delay blasting.
2. Description of the Prior Art
The art of delay blasting is practiced widely in underground and open-work blasting operations as a means of improving rock fragmentation and displacement; providing greater control of vibration, noise, and fly rock; reducing the powder factor; and reducing blasting costs. Short-interval or millisecond-delay detonators (e.g., detonators having nominal delay times of no greater than about 1000 milliseconds) and long-interval delay detonators (e.g., those having nominal delay times of greater than about 1000 milliseconds) have been designed around the needs of different blasting requirements. At the present time, millisecond (MS) delays are the most widely used delay detonators for quarry, open-bit, and construction projects, and they are also used in underground mines for multiple-row slabbing blasts, stope blasts, and other production blasts where rows of holes are breaking to a free face. Typically, MS delay blasts will move rock farther away from the face than long-interval delay blasts because of the interaction between successive boreholes fired at the shorter delay intervals. The nominal time interval between periods of successive detonators in an available series often is as low as 25 milliseconds for lower-delay-period MS detonators, although it can be up to 100 milliseconds for higher-delay-period MS detonators, and up to about 500-600 milliseconds for long-interval delay detonators.
An important prerequisite to successful delay, especially MS delay, blasting is that the delay times of a number of detonators of stated delay rating be as uniform as possible from detonator to detonator. Desirably, the variation from the nominal value of the delay times of a given group of detonators of assigned nominal delay time should be small enough that no less than 8 ms elapse between the firing of detonators of any two consecutive periods. This would mean a maximum variation of .+-.8 ms for detonators in the 25-ms; .+-.21 ms for those in the 50-ms; and .+-.46 ms for those in the 100-ms interval series. Without good uniformity, it is difficult to achieve a desired fragmentation, vibration reduction, etc. as expected from a given delay pattern.
In delay detonators, the delay interval, i.e., the time between the application of electrical or percussive energy and the detonation, is provided by the interposition of a delay charge of an exothermic-burning composition between the ignition system and the priming charge of heat-sensitive detonating explosive. The burning rate of the delay composition and the length of its column determine the delay interval. While in some detonators the delay charge is pressed, without any surrounding element, directly into the detonator shell over the primer charge, usually the delay charge is housed within a heavy-walled rigid carrier tube, e.g., as shown in U.S. Pat. Nos. 2,999,460 (FIG. 1) and 3,021,786 (FIG. 2), or in a special plastic capsule or tube as is shown in co-pending U.S. patent application Ser. No. 77,718, filed Sept. 21, 1979. The latter shows that a polyolefin or polyfluorocarbon carrier for a delay charge is advantageous in that it reduces the variability of the delay timing with changes in the surrounding temperature or medium (e.g., air vs. water).
A shorter delay interval can be provided by reducing the length of a given delay charge or using a faster-burning composition. If it is desired to produce shorter delays without resorting to changing the delay composition, uniformity of delay timing may become difficult to achieve to a degree dependent somewhat on the internal structure of the detonator and the manner in which its delay element is produced. This difficulty arises because inaccuracies in loading the small amounts of powder in the detonator shell or delay tube or capsule are common, and while a given deviation from the intended charge size or load in a given group of detonators may produce a variation from the assigned nominal delay times which is tolerable in higher-delay-period detonators, the variation produced by the same deviation in the lowest-delay-period detonators may be so great that the minimum amount of time does not elapse between the firing of detonators of any two consecutive periods. Delay detonators are needed whose delay interval is less sensitive to the small variations in delay charge size encountered in normal manufacturing processes, e.g., variations on the order of about .+-.0.03 gram.
In non-electric blasting systems, detonating cords are used to convey or conduct a detonation wave to an explosive charge in a borehole from a remote area. One type of detonating cord, known as low-energy detonating cord (LEDC), has an explosive core loading of only about 0.1 to 2 grams per meter of cord length. Such a cord is characterized by low brisance and the production of little noise, and therefore is particularly suited for use as a trunkline in cases where noise has to be kept to a minimum, and as a downline for the bottom-hole priming of an explosive charge.
In blasting practice, an LEDC downline may be joined to a delay detonator attached to the blasting explosive charge in a borehole. Detonation of the LEDC actuates the detonator, which in turn initiates the explosive charge. At the surface, a delay detonator may be interposed between two lengths of LEDC trunkline to provide a surface delay. Also, if the LEDC is of a type which is incapable of "picking up", i.e., detonating, from the detonation of a donor cord with which it is spliced or knotted, e.g., to connect downlines to a trunkline, a delay detonator may be interposed between the trunkline and downline to act as a delay "starter" for the downline.
The most desirable cord-initiated detonators are those which do not require connection to the cord at the place of manufacture. A field-assembled detonator/cord system offers such advantages as safety and convenience during handling and storage, possible separate classificaton of the components for transportation, etc.
Co-pending U.S. patent application Ser. No. 177,210, which is a continuation-in-part of now-abandoned application Ser. No. 15,288, filed Feb. 26, 1979, describes a delay detonator adapted to be assembled in the field with a length of LEDC which is placed in coaxial position in an open cavity in the detonator, thereby making the detonator particularly useful as an in-hole delay initiator when connected to an LEDC downline.
U.S. Pat. No. 3,709,149 also describes a delay detonator adapted to be assembled in the field with a length of LEDC, which is disposed outside a closed shell that contains an impact-sensitive ignition composition held, for example, in an empty primed rim-fired or center-fired rifle cartridge casing used as an end closure for the detonator. The end or side of the cord is in direct and abutting contact with the exterior surface of the primer end, thereby permitting utilization of either the side or end output of the cord for ignition. This detonator generally is positioned in a booster unit embedded in an explosive charge in a borehole.